To investigate the transient evolutions of the physical processes is always very interesting in many fields, such as physics, chemistry, biomedicine, and so on. To capture images of these physical processes accurately allows us to disclose their dynamic laws thereby to control or utilize the processes. Different processes may have different temporal characteristics. Correspondingly, to image them needs different time resolutions. An optical image may be required a time resolution with an order of magnitude of millisecond or sub-millisecond for the transient processes, e.g. the collisions between some particles; an order of magnitude of microsecond or sub-microsecond for some processes, e.g. some explosions or shock waves; an order of magnitude of nanosecond or sub-nanosecond for the high-voltage discharge and laser flying events; an order of magnitude of picosecond or sub-picosecond for the decay and migration of phonons and excitons in solids, the phase demodulation time and molecular vibrational relaxation in liquids, and growth and decay processes of plasma in gases and solids; even an order of magnitude of femtosecond or sub-femtosecond for molecular structure dynamics (atomic motions on atomic scales, such as vibrations, breaking and forming of chemical bonds, and so on). Recent researches show the movements of high-energy ions and thermal energy electrons, movements of valence electrons in a molecular, and dynamics of electrons in an atomic shell have their time scales with an order of magnitude of attosecond or sub-attosecond.
Besides the time resolution, a high spatial resolution for imaging is also very important. The aforementioned transient processes can be divided into two types: one type of transient processes occurs periodically and repeatedly, and the other works in single-shot mode, or with very low repetition rates. The former can be recorded by the pump-probe technology with a high time resolution which is determined by pulse durations of the probe beam. The development of the ultrafast laser technology has already pushed the time resolution to femtosecond, even attosecond level. For the single-shot processes, or those with very low repetition rates, to record these processes requires a high time resolution, a large frame rate and frame number. It is obvious that the pump-probe technology cannot work for real-time optical imaging.
It is well known that the key technical parameters of ultrafast framing imaging for the single-shot transient events include spatial resolution, time resolution, framing rate, framing number, and so on. Besides illumination wavelengths, high spatial resolution also depends on the transfer function of an optical imaging system. High time resolution depends on the shutter time for imaging. High framing rate is also very important for recording ultrafast events. If the framing images propagate collinearly, the framing rate is restricted by the response speed of the used recording medium. Scan recording can get rid of this restriction, however, it brings with another restriction, that is, the scan speed. Thus far, a framing frequency realized by the scan method is difficult to exceed 109 fps.